1. Field of the Invention
This invention relates to internally manifolded and internally manifolded and internally reformed fuel cell stacks, and in particular, subassemblies of an anode/current collector/separator plate/current collector/cathode therefor which upon assembly with electrolyte provide wet seals between the electrolyte and the electrodes. The subassemblies provide ease of assembly resulting in reduced labor costs and long term stability and the separator plate design reduces the amount of material required for fabrication, in particular, of the main and feed rails comprising the wet seals, resulting in reduced material costs. In accordance with one embodiment, the separator plate design provides integration of the current collector and main rail and elimination of a separate feed rail.
This invention is particularly applicable to molten carbonates and solid conductor/solid oxide fuel cells.
Generally, fuel cell electrical output units are comprised of a stacked plurality of individual cells separated by inert or hi-polar electronically conductive ferrous metal separator plates. Individual cells are sandwiched together and secured into a single stacked unit to achieve desired fuel cell energy output. Each individual cell generally includes an anode and cathode electrode, a common electrolyte "tile" or "matrix", typically referred to as the active area components, and a fuel and oxidant gas source. Both fuel and oxidant gases are introduced through manifolds to their respective reactant chambers between the separator plate and the electrolyte tile. The area of contact between the electrolyte and other cell components to maintain separation of the fuel and oxidant gases and prevent and/or minimize gas leakage is known as the wet seal. A major factor contributing to premature fuel cell failure is corrosion and fatigue in the wet seal area. This failure is hastened by thin-film electrolyte corrosion of stainless steel surfaces of the separator plate at high temperatures and high thermal stresses resulting from differing thermal expansion characteristics between the separator plate and active area components during thermal cycling of the cell, causing weakening of the electrolyte tile structure through intracrystalline and transcrystalline cracking. Such failures permit undesired fuel and/or oxidant gas crossover and overboard gas leakage which interrupts the intended electrochemical oxidation and reduction reactions, thereby causing breakdown and eventual stoppage of cell current generation. Under fuel cell operating conditions, in the range of about 500.degree. C. to about 700.degree. C., molten carbonate electrolytes are very corrosive to ferrous metals which, due to their strength, are required for fuel cell housings and separator plates. The high temperature operation of stacks of molten carbonate fuel cells increases both the corrosion and thermal stress problems in the wet seal area, especially when the thermal coefficients of expansion of adjacent materials are different.
This invention provides fully internal manifolding of the fuel and oxidant gases to and from the individual cells of an assembled stack in a manner, due to the design of the cell components, which provides ease of assembly, long term endurance, stability of fuel cell operation, and a reduced number of individual cell components, thereby eliminating fit up problems between the various cell components.
2. Description of the Prior Art
Commercially viable molten carbonate fuel cell stacks may contain up to about 600 individual cells, each having a planar area in the order of at least eight square feet. In stacking such individual cells, separator plates separate the individual cells with fuel and oxidant each being introduced between a set of separator plates, the fuel being introduced between one face of a separator plate and the anode side of an electrolyte matrix and oxidant being introduced between the other face of the separator plate and the cathode side of a second electrolyte matrix. Due to the thermal gradients between cell assembly and cell operating conditions, differential thermal expansions, and the necessary strength of materials used for the manifolds, close tolerances and very difficult engineering problems are presented.
Conventionally, stacks of individual molten carbonate fuel cells have been constructed with spacer strips around the periphery of a separator plate to form wet seals. Various means of sealing in the environment of the high temperature fuel cell wet seal area are disclosed in U.S. Pat. No. 4,579,788 which teaches wet seal strips fabricated utilizing powder metallurgy techniques; U.S. Pat. No. 3,723,186 which teaches the electrolyte itself comprised of inert materials in regions around its periphery to establish an inert peripheral seal between the electrolyte and frame or housing; U.S. Pat. No. 4,160,067 which teaches deposition of inert materials onto or impregnated into the fuel cell housing or separator in wet seal areas; U.S. Pat. No. 3,867,206 which teaches a wet seal between electrolyte-saturated matrix and electrolyte-saturated peripheral edge of the electrodes; U.S. Pat. No. 4,761,348 which teaches peripheral rails of gas impermeable material to provide a gas sealing function to isolate the anode and cathode from the oxidant and fuel gases, respectively; U.S. Pat. No. 4,329,403 which teaches a graded electrolyte composition for a more gradual transition in the coefficient of thermal expansions in passing from the electrodes to the inner electrolyte region; and U.S. Pat. No. 3,514,333 which teaches housing of alkali metal carbonate electrolytes in high temperature fuel cells by use of a thin aluminum sealing gasket. None of the above patents deal with sealing around internal fuel and oxidant manifolds in fuel cell stacks.
U.S. Pat. No. 4,510,213 teaches transition frames surrounding the active portion of the cell units to provide fuel and oxidant manifolds to the gas compartments of the individual cells, the manifolds not passing through the separators nor the electrolyte tiles of the cells. The transition frames require complicated insulating between adjacent cells and are made up of several separate and complicated components. U.S. Pat. No. 4,708,916 teaches internal manifolding of fuel and external manifolding of oxidant for molten carbonate fuel cells in which sets of fuel manifolds pass through electrodes as well as electrolytes and separators in a central portion and at opposite ends of the individual cells to provide shortened fuel flow paths. The end fuel manifolds are in a thickened edge wall area of the separator plate while the central fuel manifolds pass through a thickened central region and sealing tape impregnated with carbonate or separate cylindrical conduit inserts are provided extending through the cathode.
Internal manifolding has been attempted wherein multiple manifold holes along opposite edges of the cell have been used to provide either co- or counter-current flow of fuel and oxidant gases. These manifold holes for fuel have been located in a broadened peripheral wet seal area along opposing edges, but the manifolds have been complicated structures exterior to the electrolyte or passing through at least one of the electrodes. However, adjacent manifold holes are used for fuel and oxidant which provides short paths across a short wet seal area and leakage of the gases as well as the necessarily broadened peripheral seal area undesirably reducing the cell active area, as shown, for example, in U.S. Pat. No. 4,769,298. Likewise, prior attempts to provide internal manifolding have used multiple manifolded holes along broadened peripheral wet seal areas on each of all four edges of the cell to provide crossflow, but again, short paths between adjacent fuel and oxidant manifolds required similar complicated structures and the holes caused leakage of the gases and further reduced the cell active area.
A fully internally manifolded molten carbonate fuel cell stack is taught by U.S. Pat. No. 4,963,442, U.S. Pat. No. 5,045,413, and U.S. Pat. No. 5,077,148. each of which teaches a separator plate for a molten carbonate fuel cell stack having a flattened peripheral wet seal structure extending to contact the electrolytes on each face of the separator plates completely around their periphery forming a separator plate/electrolyte wet seal under cell operating conditions, and having a plurality of aligned perforations surrounded by a flattened manifold wet seal structure extending to contact the electrolyte on each face of the separator plate, forming a separator plate/electrolyte wet seal under cell operating conditions. In accordance with the teachings of these patents, the separator plates are pressed metal plates in which the flattened peripheral wet seal structure and the extended manifold wet seal structure on one face of the separator plate is a pressed shaping of the separator plate and on the other face of the separator plate is a pressed sheet metal shape fastened to said other face of the separator plate. In addition, conduits through the manifold wet seal structures are provided between one set of manifolds and anode chambers on one face of the separator plates for fuel gas and between the other set of manifolds and the cathode chambers on the other face of the separator plates for oxidant. These conduits are formed by corrugated metal or holes through sheet metal structures secured to the separator plate. Thus, a separator plate for a fuel cell unit in accordance with the teachings of these patents comprises as many as nine (9) individual pieces welded together and a fuel cell unit in accordance with the teachings of these patents comprises at least five (5) pieces in addition to the separator plate, namely, cathode and anode current collectors, cathode and anode electrodes and an electrolyte. In addition, to accommodate the current collectors and electrodes within the center portion of the separator plate, the wet seal structures are in the form of steps such that the current collectors and electrodes, when disposed in the center portion of the separator plates, are flush with the top portion of the step which forms the wet seal between the separator plate and the electrolyte. Such fit up of pieces into pressed steps results in variable elevational discontinuities which are known to cause cracking of the electrolyte and result in gas crossflow through the electrolyte tiles.